1. Field of the Invention
This invention relates generally to underwater imaging, and more specifically to an apparatus and method using Laser Imaging And Ranging (LIDAR), and range gated laser imaging underwater by a helmeted diver while retaining hands free mobility, and retaining the option of viewing the real underwater world when required.
2. Brief Description of the Prior Art
Divers have long been plagued with constraints on underwater visibility, especially when working in shallow water environments, such as harbors or rivers. Furthermore, a diver's capability to adequately inspect structures underwater is reduced in murky water. Divers will report anything that they see or discover relative to their mission during the course of an inspection underwater. Inspections done in murky water limit severely the number of square feet of surface area of an inspected structure that the diver will view during a given time frame. Due to time constraints imposed on divers limiting the length of dives and the lack of geographical guideposts or markers to use as references while inspecting large structures in limited visibility conditions, the quality of an inspection is, in part, proportional to the visibility conditions of the water or fluid that the diver is immersed in.
In the past, divers have had to contend with limited visibility in murky water, while accomplishing tasks which require at least some degree of water clarity. Many tasks, such as inspections, underwater cutting and welding, and repairs to ships require some minimal amount of visibility. Tasks undertaken under this type of constraint may take up to twice as long as in clear water. This is both costly and time consuming. Topside support personnel, marine superintendents and engineers have not been able to see what the diver was doing while working in murky water.
Closed circuit television cameras have helped advance the state of the art, with the Silicon Intensified Tube (SIT) camera which is an extremely low light level camera for use in highly backscattering mediums which prohibit the use of artificial lighting. The SIT camera has its drawbacks, however, and offers visibility improvements only slightly better than using the unaided eye. In addition, ambient light levels in highly backscattering mediums are often significantly reduced by the suspended particles, and require artificial lighting to produce a clearly recognizable image. As a result, underwater communication under these constraints is reduced with often unsatisfactory results.
It is therefore often desirable for divers working in conditions of limited underwater visibility to use "LIDAR" (Laser Imaging And Ranging) or range gated laser imaging underwater to increase their identification and detection capabilities and ranges. The enhancement of object detection and discernment allows divers to complete mechanical tasks in significantly shorter periods of time than without enhancement.
Transmission of light in the ocean depends on the frequency of the light and type of water, or "Jerlov Class". The clearer the water, the lower the Jerlov Class number. For example, Jerlov Class I water reduces an incoming laser pulse to 1/e (1/2.72=0.368) of its initial intensity after passing through 190 feet (58 meters) of sea water of the Jerlov Class I. Other less clear waters will attenuate light pulses more strongly, with maximum transmission occurring at progressively longer wavelengths, and with less frequency dependence.
It is well known in the commercial diving industry that a diver will be able to accomplish a task in up to 50% less time in very clear water, such as Jerlov Class I, than in extremely murky water. However, the majority of tasks that divers are asked to accomplish take place in water visibility which is Jerlov Class III or worse.
Laser Imaging And Ranging (LIDAR), and range gated laser imaging are methods by which a very short burst of high energy light is transmitted through a backscattering medium, then a shutter is opened in an imaging device at precisely the right time to allow light returning from objects at the desired range to pass and form an image. Light returned from objects at too short a distance arrives before the shutter opens and is rejected as a result, and light reflected from objects at too long a range arrives after the shutter closes and is rejected. If this imaging is repeated at sub-second intervals (10-36 repetitions/second), a real time image is generated.
LIDAR significantly reduces the amount of backscattered light encountered in imaging fluid mediums. Since the light has already passed much of the backscattering causing suspended particles in the fluid when the imaging takes place, those particles are not illuminated and are not as discernible as when highly illuminated. The object being illuminated and a relatively small amount of backscattering particles are illuminated using LIDAR, creating a clearly discernible image at significantly greater ranges than by using any other imaging methods.
Neumann, U.S. Pat. No. 3,380,358 discloses a relatively simple range gated laser imaging system for reducing degradation of the formed image by unwanted reflections of nearer and farther objects in which a Q-switched ruby laser illuminates objects to be photographed with a 60 nanosecond illuminating pulse of energy. The shutter grid of an image converter camera tube is opened for 50 nanoseconds after an elapsed time corresponding to the travel time of the energy pulse from the laser to the object and return.
Keeler, U.S. Pat. No. 5,091,778 discloses a LIDAR system for detecting and imaging underwater objects from an airborne platform or from a submarine using tunable output wavelength frequency lasers at a selected water depth having a selected Jerlov Class associated therewith and converting the detected pulses of light to a video image. This system offers two to three times the depth penetration in seawater of prior LIDAR systems.
LIDAR systems have not been used as an imaging medium for a helmeted, surface supplied diver, in working situations which would render an image to the diver in such a way that the diver has his hands free for work while submerged. This is due in large part to the fact that there has been no available way to image a large size display directly in front of the diver's face using a slim profile screen which is removable from the divers field of view at will, and in part to the fact that LIDAR has not been adapted to this type of utility. Another major reason why this problem has not been addressed prior to this time is because the technology for manufacturing solid state liquid crystal display screens capable of displaying a large color image has not been available until about 1992.
The present invention overcomes the above discussed problems and is distinguished over the prior art in general, and these patents in particular by a hands free LIDAR imaging system for divers having a laser emitter and detector mounted in waterproof enclosures on the diver's helmet for imaging objects which the diver could not otherwise see clearly or at all, depending on their distance from the diver and clarity of the fluid medium and a hinged viewing screen attached to the diver's helmet or mask face plate that permits hands free viewing and which can be moved out of the divers field of view at will. Optimal laser wavelength output is typically in the blue region of the optical spectrum, between 460-600 nm. The laser emitter and detector are oriented so that the laser emitter illuminates areas in the divers field of view which are similar to what he sees through the viewing port of the helmet. The laser detector detects reflected laser light from objects illuminated in the same areas. The pressure and waterproof equipment is connected to a LIDAR system via electrical cables. The electronics control package, computer, timing and range gating control and signal processing equipment is remote from the diver and is not intended to be submerged in a fluid environment. The laser system may be a simple range gated laser imaging system or may be a more sophisticated LIDAR system employing tunable frequency lasers. The present system improves visibility in the fluid medium by four to seven times that of the unaided eye, which offers a tremendous improvement for task accomplishment time and detection capability.